Electrophysiology catheters are commonly-used for mapping electrical activity in the heart. Various electrode designs are known for different purposes. In particular, catheters having basket-shaped electrode arrays are known and described, for example, in U.S. Pat. Nos. 5,772,590, 6,748,255 and 6,973,340, the entire disclosures of both of which are incorporated herein by reference.
Basket catheters typically have an elongated catheter body and a basket-shaped electrode assembly mounted at the distal end of the catheter body. The assembly has proximal and distal ends and comprises a plurality of spines connected at their proximal and distal ends. Each spine comprises at least one electrode. The assembly has an axial elongated expander which is longitudinally movable relative to the catheter by an EP professional to vary the configuration of the basket between an expanded arrangement wherein the spines bow radially outwardly and a collapsed arrangement wherein the spines are arranged generally along the axis of the catheter body. The catheter may further comprise a distal location sensor mounted at or near the distal end of the basket-shaped electrode assembly and a proximal location sensor mounted at or near the proximal end of the basket-shaped electrode assembly. In use, the coordinates of the distal location sensor relative to those of the proximal sensor can be determined and taken together with known information pertaining to the curvature of the spines of the assembly to find the positions of the at least one electrode of each spine.
A basket-shaped electrode assembly is capable of detecting in a single beat most or all of the electrical function of the left or right atrium. However, because the atria of an individual patient may vary in size and shape, it is desirable that the assembly be sufficiently versatile and steerable to conform to the particular atrium. A basket catheter with a deflectable assembly for improved maneuverability to provide better tissue contact, especially in a cavernous region of the heart, including an atrium, is described in U.S. Pat. No. 9,204,929 (corresponding to application Ser. No. 14/028,435, filed Sep. 16, 2013), the entire disclosure of which is hereby incorporated by reference.
While a deflectable basket catheter whose basket configuration can be varied by an expander enables an EP professional to adjust the basket for a better fit within any particular atrium, a basket with stiffer spines may enable better contact between the spines and the atrial wall. However, stiffer spines may increase the risk of injury and damage to the atrial wall.
Nitinol wire is often used in the construction of therapeutic and diagnostic catheter distal ends, including basket-shaped electrode assemblies. At body temperature, nitinol wire is flexible and elastic and like most metals nitinol wires deform when subjected to minimal force and return to their shape in the absence of that force. Accordingly, a 3-D distal assembly can be easily collapsed to be fed into a guiding sheath, and readily deployed in the chamber or tubular region upon removal of the guiding sheath. Because Nitinol belongs to a class of materials called Shaped Memory Alloys (SMA). These materials have interesting mechanical properties beyond flexibility and elasticity, including shape memory and superelasticity which allow nitinol to have a “memorized shape.”
Nitinol has different temperature phases, including martensitic phase and austenite phase. The austenite phase is Nitinol's stronger, higher-temperature phase. Crystalline structure is simple cubic. Superelastic behavior occurs in this phase (over a 50°-60° C. temperature spread). The Martensite phase is Nitinol's weaker, lower-temperature phase. Crystalline structure is twinned. Material is easily deformed in this phase. Once deformed in martensite, it will remain deformed until heated to austenite where it will return to its pre-deformed shape, producing the “shape memory” effect. The temperature at which Nitinol starts to transform to austenite upon heating is referred to as the “As” temperature. The temperature at which Nitinol has finished transforming to austenite upon heating is referred to as the “Af” temperature.
Accordingly, it is desirable that a basket catheter have spines that are sufficiently pliable and flexible to minimize the risk of injury and damage to the atrial wall, yet provide sufficient stiffness for dependable tissue contact and electrode spacing. It is also desirable that a basket catheter have spines constructed of material with shape memory, such as nitinol, so as to employ some of its advantageous properties.